An electronic imaging system typically produces a signal output corresponding to a viewed object by spatially sampling an image of the object in a regular pattern with an array of photosensitive elements, such as, for example, with a charge-coupled device (CCD) solid-state image sensor. In such an imaging system, it is well-known that detail components in the object which contain frequencies too high to be analyzed within the sampling interval of the sensor contribute to the amplitudes of lower frequency components, and thereby produce imaging errors commonly referred to as aliasing or undersampling artifacts. In particular, if spatial detail being imaged contains a high frequency component of a periodicity smaller than the pitch (periodicity) of each neighboring photosensitive picture element of the solid state image sensor, the subsequent detection of this high frequency component tends to result in a spurious signal due to aliasing.
In general, the electronic imaging system can minimize aliasing if its optical section has a frequency response that cuts off, or filters out, the higher frequency content of the object. As a result, the optical section generally employs an optical low pass filter to substantially reduce the high frequency component contained in the spatial detail of the image received by the image sensor. It is thus well-known in the prior art that the design of electronic imaging systems involves a trade-off between image sharpness and the susceptibility of the imaging system to aliasing distortions or undersampling artifacts.
To limit these artifacts, an optical filter such as, for example, a birefringent blur filter has become a common component in consumer color video cameras. U.S. Pat. Nos. 4,101,929 and 4,896,217 show typical examples of such filters. Such a filter is typically placed between a lens and the image sensor to provide a low-pass filter function which reduces the spatial frequency content of the object at frequencies above the Nyquist frequency of the photosensitive elements. This makes the imaging system less susceptible to aliasing distortion. For example, for many available sensors wherein equal pixel densities in each of the sensed colors provide that each of the sensed colors have the same Nyquist frequency, an achromatic low-pass, or "blur", function is effective in minimizing aliasing distortion. Such a function can be provided by a birefringent filter.
The birefringent blur filter is typically composed of filter plates manufactured from a crystalline material like quartz that exhibits a dual refraction effect when the crystal axes of the filter plates are oriented at an angle with respect to the plate surface. In this orientation, a randomly polarized ray of light passing through such a filter plate emerges as two separated polarized rays. The combination of several of such plates produces a multiple spot pattern from each incident point in the image. If this spot pattern distributes light energy over multiple photosensitive elements, then the effect of a blur is obtained. This will limit the optical transfer function of the system at spatial frequencies above the Nyquist frequency of the photosensitive elements. However, this type of filter suffers from the drawback that it is costly and complicated to manufacture. In addition, a practical birefringent filter tends to be rather large and thick. Indeed, the thickness required to achieve the desired blur requires a lens with a long back focal length in order to make room for the blur filter in the optical path. Space limitations often do not allow such an optical structure, and lens design becomes unduly complicated. Finally, since such a filter requires randomly, or nonpolarized, light, a polarizing filter cannot be allowed in such a system to obtain well known photographic polarizing effects.
It is also well known in the art to use a phase diffraction grating as a frequency selective filter to produce an image blur. For example, as shown in U.S. Pat. No. 4,998,800, the periodicity of an image of a diffraction grating projected onto a solid state image sensor is selected to be a multiple of the periodicity of the photosensitive picture elements, and a blurred image is obtained. This type of filter, however, suffers from the drawback that, instead of producing a tightly controlled pattern over a few photosensitive elements, it spreads light over many interference fringes (orders) theoretically out to infinity. In addition, it is difficult to control the energy distribution in the fringes in order to obtain an acceptable blur function covering a designated number of pixels. Moreover, the energy distribution is dependent upon wavelength.
As can be appreciated from the foregoing remarks, there is a need in the art for a physically small blur filter that is inexpensive and relatively simple to manufacture, yet produces a tightly controlled blur pattern that is not dependent upon polarization techniques. As an alternative to the birefringent blur filter and the phase diffraction grating, U.S. Pat. No. 4,989,959 discloses a pyramidal blur filter comprised of four wedges which divide incident light into four quadrants so that light from the same image point impinges, on the average, on several photosensitive elements in the image sensing device. A further alternative is described in Ser. No. 040,948 now U.S. Pat. No. 5,322,998, entitled "A Conical Blur Filter for Reducing Artifacts in Imaging Apparatus", which was filed on even date herewith by common assignee. In such imaging apparatus, an optical blur filter having a substantially conical shape is used to image a circular blur pattern upon an array of adjacent photosensitive elements. Both the pyramidal and the conical optical blur filters, however, suffer from the drawback that the size of the blur pattern is dependent upon the focal length of an imaging lens that is used in the optical section of the apparatus. This makes it difficult to use the apparatus with, for example, a zoom lens.